Photoacids are

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

s which become more

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

ic upon absorption of light. Either the light causes a

photodissociation Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...

to produce a

strong acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions. ...

or the light causes photoassociation (such as a ring forming reaction) that leads to an increased acidity and dissociation of a proton. There are two main types of molecules that release protons upon illumination: photoacid generators (PAGs) and photoacids (PAHs). PAGs undergo proton photodissociation irreversibly, while PAHs are molecules that undergo proton photodissociation and thermal reassociation. In this latter case, the

excited state In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers to a ...

is strongly acidic, but reversible.

Photoacid generators

An example due to photodissociation is triphenylsulfonium triflate. This colourless salt consists of a

sulfonium In organic chemistry, a sulfonium ion, also known as sulphonium ion or sulfanium ion, is a positively-charged ion (a " cation") featuring three organic substituents attached to sulfur. These organosulfur compounds have the formula . Together wi ...

cation and the

triflate In organic chemistry, triflate (systematic name: trifluoromethanesulfonate), is a functional group with the formula and structure . The triflate group is often represented by , as opposed to −Tf, which is the triflyl group, . For example, ' ...

anion. Many related salts are known including those with other

noncoordinating anion Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found ...

s and those with diverse substituents on the phenyl rings. The triphenylsulfonium salts absorb at a wavelength of 233 nm, which induces a dissociation of one of the three

phenyl In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen ...

rings. This dissociated phenyl radical then re-combines with remaining diphenylsulfonium to liberate an H⁺ ion. The second reaction is irreversible, and therefore the entire process is irreversible, so triphenylsulfonium triflate is a photoacid generator. The ultimate products are thus a neutral organic sulfide and the strong acid

triflic acid Triflic acid, the short name for trifluoromethanesulfonic acid, TFMS, TFSA, HOTf or TfOH, is a sulfonic acid with the chemical formula CF3SO3H. It is one of the strongest known acids. Triflic acid is mainly used in research as a catalyst for este ...

. : C₆H₅)₃S⁺CF₃SO] + ''h''ν → C₆H₅)₂S^+.CF₃SO] + C₆H : C₆H₅)₂S^+.CF₃SO] + C₆H → (C₆H₅C₆H₄)(C₆H₅)S + Triflic_acid.html" ;"title="F₃SO">Triflic acid"> F₃SOH⁺ Applications of these photoacids include photolithography and catalysis of the polymerization of epoxides.

Photoacids

An example of a photoacid which undergoes excited-state proton transfer without prior photolysis is the fluorescent dye pyranine (8-hydroxy-1,3,6-pyrenetrisulfonate or HPTS). The Förster cycle was proposed by

Theodor Förster Theodor Förster (May 15, 1910 – May 20, 1974) was a German physical chemist known for theoretical work on light-matter interaction in molecular systems such as fluorescence and resonant energy transfer. Education and career Förster studied ...

{{cite journal , title=The Scientific Work of Theodor Förster: A Brief Sketch of his Life and Personality , journal = ChemPhysChem, volume = 12, issue = 3, pages = 555–558, last1=Kramer , first1=Horst E. A. , last2=Fischer , first2=Peter , date=9 November 2010 , doi=10.1002/cphc.201000733 , pmid = 21344592 and combines knowledge of the ground state

acid dissociation constant In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction :HA ...

(pK_a), absorption, and fluorescence spectra to predict the pK_a in the excited state of a photoacid.

References

Photochemistry Lithography (microfabrication) Microtechnology Light-sensitive chemicals Acids